Current technology permits the translation of analog audio signals into a sequence of binary numbers. These numbers can then be transmitted through a variety of different transmission facilities and then can be converted back into analog audio signals. The device for performing both the conversion from analog to binary and the conversion from binary back to analog is called a CODEC. This is an acronym for Coder/DECoder.
The cost of transmitting bits from one location to another is a function of the number of bits transmitted per second. The higher the bit transfer rate the higher the cost, Certain laws of physics and psychoacoustics describe a direct relationship between perceived audio quality and the number of bits transferred per second. The net result is that improved audio quality increases the cost of transmission. CODEC manufacturers have developed technologies to reduce the number of bits required to transmit any given audio signal (compression techniques) thereby reducing the associated transmission costs. The cost of transmitting bits is also a function of the transmission facility used, i.e. satellite, PCM phone lines, ISDN, ATM.
A CODEC that utilizes some of these compression techniques also acts as a computing device. The CODEC inputs the analog audio, converts the audio to digital bit streams, and then applies a compression technique to the bits thereby reducing the number of bits required to successfully transmit the original audio signal. The receiving CODEC applies the same compression techniques in reverse (decompression) so that it is able to convert the compressed bit stream back into analog audio output. The difference in quality between the analog audio input and the reconstituted audio output is a measure of the quality of the effectiveness of the compression techniques utilized. The highest quality technique would yield an identical signal reconstruction.
Currently, the most successful audio compression techniques for general audio sounds (as opposed to human speech sounds) are called perceptual coding techniques. These types of compression techniques attempt to model the human ear. These compression techniques are based on the recognition that much of what is given to the human ear is discarded (masked) because of the characteristics of the human hearing process. For example, if a loud sound is presented to a human ear along with a softer sound, the ear will hear only the louder sound. Whether the human ear will hear both the loud and soft sounds depends on the frequency of each of the signals. As a result, encoding compression techniques can effectively ignore the softer sound and not assign any bits to its transmission and reproduction under the assumption that a human listener can not hear the softer sound even if it is faithfully transmitted and reproduced.
All perceptual coding techniques have certain parameters that determine their behavior. For example, the coding technique must determine how soft a sound should be relative to a louder sound in order to determine whether the softer sound would be masked and could then be excluded from transmission. A number that determines this masking threshold is considered a parameter in the compression technique. These parameters are largely based on the human psychology of perception, so they are collectively known as psycho-acoustic parameters.
In order to ensure interoperability of CODECs from different manufacturers and to ensure an overall level of audio quality, standard coding techniques have been developed. One such technique is the so-called ISO/MPEG Layer-II compression standard. This technique or standard is a process for the compression and decompression of an audio input. This standard dictates a bit stream syntax for the transmission of the binary data after it is compressed and for the compression technique itself. Further, the standard includes a collection of psycho-acoustic parameters that is useful in performing the compression. U.S. Pat. No. 4,972,484, entitled "Method of Transmitting or Storing Masked Sub-band Coded Audio Signals," discloses the ISO/MPEG Layer II technique operable in the CODECs of different manufacturers.
Current standards, however, do not require any specific parameter set. The manufacturers of CODECs determine a set of psycho-acoustic parameters either from the standard or as modified by the manufacturer in an attempt to provide the highest quality sound with the lowest number of bits. Once a given parameter set is determined, the manufacturer selects what is perceived as the best value for each of the parameters, and that set of values determines the resultant quality of the CODEC's audio output. Presumably, a given manufacturer will choose a parameter set to provide what it perceives as the best resultant quality. In currently available CODECs, users typically are unaware of the existence or nature of these parameters. The user has no control over the actual parameters even though they directly affect the quality of the audio output. As a result, the users must test different CODECs from different manufacturers and then select the one device that meets requirements or sounds best to the particular user.
Although no set parameters are required, ten (10) standard parameters are typically included in prior art CODECs. These prior art CODECs have implemented these 10 standard parameters because they have been accepted by the ISO and have been adopted as part of the ISO/MPEG Layer-H compression standard. This standard and its utilization of the 10 parameters does not utilize or provide CD quality output that the user desires.
The applicant has discovered that this is a problem because the value for each standard parameter is determined based on the average human ear. The parameters do not take into account the variations between each individual's hearing capabilities. The applicant has recognized that in existing CODECs, no method or apparatus is available for users to tune their CODECs to address these subjective criteria and meet changing audio needs and to shape the overall sound of their application. Accordingly, a user must test different CODECs from different manufacturers and then select the one device that has the features or options they desire. The applicant has also discovered that the inclusion of other parameters can provide closer to CD quality sound than a CODEC that includes only the 10 standard parameters. Applicant has also discovered that adjustment of these additional parameters can further improve the quality of the resultant audio output.